A scanner is an image input device that optically scans a medium such as a document, a drawing, a photograph, a film or the like and inputs a digital image by converting analog signals indicative of the intensity of the light reflected from or passed through the medium to digital signals.
In general, a user of a scanner brings a scan target such as a document or the like onto a transparent scan region of a scanner and covers the scan region with a cover for cutting an external disturbing light. If a switch is pressed subsequently, an image sensor, e.g., a one-dimensional array of charge coupled devices (CCDs) or amorphous semiconductors, is parallel-moved so that the scanner can two-dimensionally scan one surface of the medium.
In case of a medium having a size of a credit card, e.g., an identification card or a name card, one surface of the medium is scanned while loading the medium to a scanner whose image sensor is kept stopped.
However, in a process requiring a high processing speed, e.g., an immigration inspection process in which a passport presented by a tourist is scanned to read a passport number or the like written in a machine-readable zone of the passport, it is cumbersome to close a scanner cover and to press a switch, which may significantly reduce the work efficiency.
In an effort to solve this problem, U.S. Pat. Nos. 6,753,982 and 5,568,281 disclose a scanner that includes an additional means capable of automatically detecting whether a scan target is positioned in a scan region.
However, the scanner of the patents cited above is complex in structure and costly to manufacture because of the provision of the additional detecting means. If the scanner is used without closing the cover, there is a possibility that, under the influence of natural light or artificial illumination light, a scan target not positioned in a scan region may be recognized as if it is positioned in the scan region. In addition, it is not easy to confirm whether a scan target is positioned in a right position.